The present invention relates to an image processing apparatus, and more particularly to an apparatus for quantizing a color image.
For processing a color image, it has been customary to produce color-separated images of three colors such as red, green, and blue or yellow, magenta, and cyan from the original color image, and then process the color-separated images. One well known process for quantizing the color-separated images is a minimized average error method. According to this process, the pixels of each color-separated image are successively extracted by main and auxiliary scannings in prescribed directions, and the extracted pixels in question are quantized as follows: First, evaluation pixels which are located in the vicinity of a pixel in question and which have already been quantized are extracted. Then, quantizing errors of the respective evaluation pixels are weighted and totaled. Each element of a weighting matrix as shown in FIG. 2 of the accompanying drawings is widely used to provide weighting values in such a weighting process. The reference letter A in FIG. 2 indicates the pixel in question. Then, the pixel in question is quantized so that the sum of the weighted quantizing errors of the evaluation pixels and a similarly weighted quantizing error of the pixel in question will be minimal. For example, for binarizing a pixel positioned at coordinates (x, y), a threshold value T is determined as follows: ##EQU1## where E(x, Y) represents a quantizing error at the coordinates (x, y); and .alpha..sub.kl, each element of the weighting matrix shown in FIG. 2. Then, the pixel in question is binarized according to the threshold thus determined.
Inks of yellow, magenta, and cyan used in color printers and the like do not have ideal spectral characteristics, but contain other color components. Therefore, eight colors (including a background color) which are produced by using the three-color inks singly or overlappingly do not exhibit ideal spectral characteristics as shown in the table of FIG. 3. And the special characteristics of each color vary nonlinearly. For the reasons described above, a reproduced color image which has been processed in the above manner differs in color tone from the original color image.